What AS number to use?
In addition to overseeing IP address allocation and Domain Name Systems (DNS), the Internet Assigned Numbers Authority (IANA) assigns public AS numbers. The public AS numbers are from 1 to 64,511. The ASNs 0, 54272–64511, and 65535 are reserved by the IANA. These ASNs should not be used.
ASNs are assigned in blocks by the Internet Assigned Numbers Authority (IANA) to Regional Internet Registries (RIRs) who then assign ASNs to companies within that RIRs geographic area. Usually these companies are ISPs, and to receive an ASN you must complete the application process of the local RIR and be approved before being assigned an ASN. The RIRs names and regions are:
AFRINIC Serves the African continent
APNIC Asia-Pacific including China, India, and Japan
ARIN American registry including Canada and United States
LACNIC Latin America, including Mexico, Caribbean, Central and South America
RIPE NCC Europe, the Middle East, former USSR, and parts of Central Asia
AS numbers from 64512 to 65534 are reserved for private use. Private AS numbers can be used for any internal networks with no outside connections to the Internet such as test networks, classroom labs, or other internal-only networks that do not access the outside world. You can also configure border routers to filter out any private ASNs before routing traffic to the outside world. If you must use private ASNs with public networks, this is the only way
to configure them. However, it is risky because many other private networks could be using the same ASNs and conflicts will happen. It would be very much like your local 192.168.0.0 network being made public — the resulting problems would be widespread.
In 1996, when RFC 1930 was written only 5,100 ASes had been allocated and a little under 600 ASes were actively routed in the global Internet. Since that time many more public ASNs have been assigned, leaving only a small number. For this reason 32-bit ASNs (four-octet ASNs) were defined to provide more public ASNs. RFC
4893 defines 32-bit ASNs, and FortiGate units support these larger ASNs.
Area border router (ABR)
Routers within an AS advertise updates internally and only to each other. However, routers on the edge of the AS must communicate both with routers inside their AS and with routers external to their AS, often running a different routing protocol. These routers are called Area Border Routers (ABRs) or edge routers. Often ABRs run multiple routing protocols to be able to redistribute traffic between different ASes that are running different protocols, such as the edge between an ISP’s IS-IS routing network and a large company’s OSPF network.
OSPF defines ABRs differently from other routers. In OSPF, an ABR is an OSPF router that connects another AS to the backbone AS, and is a member of all the areas it connects to. An OSPF ABR maintains a LSA database for each area that it is connected to. The concept of the edge router is present, but its the edge of the backbone instead of the edge of the OSPF supported ASes.
Neighbor routers
Routing involves routers communicating with each other. To do this, routers need to know information about each other. These routers are called neighbor routers, and are configured in each routing protocol. Each neighbor has custom settings since some routers may have functionality others routers lack. Neighbor routers are sometimes called peers.
Generally neighbor routers must be configured, and discovered by the rest of the network before they can be integrated to the routing calculations. This is a combination of the network administrator configuring the new router with its neighbor router addresses, and the routing network discovering the new router, such as the hello packets in OSPF. That discovery initiates communication between the new router and the rest of the network.